This invention pertains to flywheel energy storage systems and more particularly to an improved inductor alternator flywheel system that eliminates power disruptions, has increased efficiency, and is more compact than previous devices. The higher efficiency of the invention allows its proficient operation with an activated field power, thus eliminating short-term interruptions of power occurring with prior systems and also potentially for allowing higher speed and higher power operation.
Flywheel energy storage systems are being employed for supplying power to a protected load during interruptions of power from the grid. Flywheel systems use a large inertia flywheel that is supported by a bearing system and is coupled to a motor/generator. The motor/generator converts between electrical and mechanical energy by accelerating the flywheel for storing energy and decelerating the flywheel for retrieving energy. The flywheel is typically housed in a low-pressure container to limit losses from aerodynamic drag. Such systems are very advantageous for supplying back-up power to communications networks during power outages, and can also be used as ride-through devices that supply power during the start-up period that an auxiliary power source, such as a generator, needs before it can come on-line. Flywheels offer very high power capability and increased reliability over conventional electrochemical batteries, and also have longer lifespans.
To date, various types of flywheel systems have been proposed, each having different goals and attributes for their different applications. One type of flywheel system that is currently being utilized employs an inductor alternator for the motor/generator. In general, alternators use field coils that create a magnetic field which can be varied by controlling power to the field coil, and thereby control the output voltage that is generated from rotation of an armature in the magnetic field. This control can be used to maintain a relatively constant desired output voltage for a flywheel system as the speed of the flywheel slows. In contrast, conventional permanent magnet generators suffer from an output voltage that decreases linearly with speed, necessitating the use of output power conversion electronics to maintain the desired constant output voltage. For very high power systems, the cost of power electronics increases substantially, thus making an alternator flywheel system desirable.
Inductor alternators are a type of alternator particularly well suited for flywheel and other high speed applications since they have both the field and armature coils stationary. This eliminates the need to reinforce coils for high-speed rotation and also the need for electrical contact with the rotating flywheel that would require brushes that wear. Inductor alternators generate power by inducing a voltage in the armature windings from periodic changes in the magnetic flux that links the armature coils. Current in the field windings sets up the flux, and changes in the flux through the armature windings are the result of changes in the reluctance of the magnetic circuit. The rotor of the inductor alternator causes these changes by rotation of teeth or poles in an air gap in the magnetic circuit, thereby increasing and decreasing the air gap as the teeth or poles pass through it. The rotor of an inductor alternator can thus be made very simply as a gear and as such is well suited for high-speed rotation When the rotor of an inductor alternator is combined into a flywheel, the resulting device can be well suited for both storing energy and for delivering a high power constant output voltage.
One such inductor alternator flywheel system of the prior art is shown in FIG. 1. The flywheel system 30 is comprised of a solid steel flywheel rotor 31 that rotates inside a shell 40. The flywheel 31 has integral shafts 36 that journal the rotor inside upper and lower mechanical bearings 37 and 38. The flywheel rotor 31 stores the kinetic energy and also serves as an inductor alternator rotor by having an outer surface defined by poles 32, each having a radius 34, separated by recesses 33 with radii 35. A stationary field coil 39 generates flux, which flows axially into the flywheel 31, then radially outward through the poles 32 and through armature coils 43. A laminated ring 42 diffuses changes in the flux created from rotation of the flywheel poles 32. The outer shell 40 that contains the vacuum also serves as a path for the flux to connect back to the field coil 39 and complete the magnetic circuit.
Although this type of inductor alternator flywheel system is relatively simple and is capable of supplying high levels of power, this system has several significant drawbacks. The system has power losses that generate heat during standby operation while rotating at full speed with no loss of primary power. To alleviate these losses, the power to the field coil is kept reduced when in standby mode. This reduces the flux through the rotor and its poles and the armature coils. In standby operation and also when the flywheel is accelerating, the armature coils are energized by synchronized multiphase power (typically 3 phase), to make the inductor alternator function as a motor. Energizing of a set of coils causes the rotor to rotate such that its poles or teeth tend to line up with the energized coils and when they do, the sequentially next coils are energized to propel it around. In the process, the energizing of armature coils and the rotation of rotor poles into a changing magnetic field, the magnetic flux through the rotor poles continuously changes. Changes in flux through any conductor, such as the steel rotor, cause generation of eddy currents. The large teeth allow a very big area for generation of large eddy currents. The greater the currents, the greater the loss and heat created. Losses also occur from the changing flux in the rotor due to magnetic hysteresis. Losses for the system shown in FIG. 1 during standby operation are approximately 2 kilowatts. If the field coil is fully energized and the system is discharging, losses can expectedly be much higher. Removal of large amounts of continuous heat requires fans and periodic maintenance for cleaning air filters. More importantly, current systems operate at low tip speeds, stress levels and energy storage capability. The radial and hoop direction stresses for a 25 inch diameter flywheel rotating at 7000 rpm are shown in FIGS. 2A and 2B. As can be seen, the stress levels at this operating speed are low and the energy storage per the size of flywheel is low. If such flywheel inductor alternators were rotated to higher speeds to increase the energy storage density, the eddy current losses would increase with the square of the speed.
In addition to these high losses and low storage issues, another problem with the system shown in FIG. 1 operating in stand-by mode with reduced power to the field coil is that there is a time lag before it can provide full power in the event of a power interruption. A monitoring system is used to monitor the primary power. When it senses an interruption of primary power, it sends a drive signal to the field coil in the flywheel storage unit that causes the field coil current to ramp up such that required output power is provided. Some amount of time is required between sensing the loss of power and increasing the field coil power to raise the output of the system to full power. For some critical applications such as in telecommunications, computers and semiconductor manufacturing, loss of power for even fractions of a second may be costly.
One well-known method for reducing losses in motor/generators and other magnetic circuit devices is to fabricate the magnetic path with laminated construction. The laminated construction builds the structure by stacking multiple layers of the magnetic circuit material together, each layer being electrically insulated from adjacent layers through use of insulative coatings. This construction reduces eddy current losses because eddy currents generated are limited in size to flow only in the thickness on the individual lamination layers. Loss reduction in motor/generators is also helped by specially selection the material of the magnetic path and its heat treatment. The material and condition are usually made to increase the magnetic permeability of the material, decrease its electrical conductivity and to reduce hysteresis losses. These steels typically have very low carbon contents. Different types of lamination or electrical steels can be selected based on the application, acceptable losses and acceptable costs. Silicon steels are very commonly used because the added silicon decreases the volume resistivity for further reduction of eddy current losses. Nickel alloys containing between 49% to 80% nickel with the remainder being nearly pure iron are used because of their very high permeability at low inductions and very low core losses. However, these alloys are very expensive. All types of lamination steels can be used to reduce the losses in inductor alternators. Unfortunately, lamination steels and their preferred heat treat condition for achieving high efficiency are not optimal for high speed operation with high stress levels. The strength of steels is directly related to its hardness condition. The maximum hardness that can be achieved with a given steel is directly related to the amount of carbon in the steel. These steels purposely have very low to almost no carbon in them. Further reducing the strength of these materials is the choice of heat treat condition for low losses. Lamination steels are used in the annealed condition. Annealing of steels results in the lowest possible strength. This is the opposite of quenching and tempering heat treatment preferably used to develop high strength for a high speed, highly stressed flywheel.
A high speed reluctance generator with a laminated rotor and stator of prior art is shown in FIG. 3. The reluctance motor/generator 50 is comprised of a rotor 53 that is fabricated from multiple axially stacked laminations 55, attached to a central shaft 56. A laminated stator 52, which also has poles 54, surrounds the rotor 53. The radial distance between the rotor 53 and stator 52 defines an air gap 57 of the machine. This use of the laminations allows for operation to very high rotational speeds, contemplated up to 100,000 rpm. The laminations reduce the losses such that a high frequency magnetic field from such high rotational speeds can be achieved. Although, the laminations in this rotor allow high rotational speeds, the rotor is not known to be capable of achieving high peripheral speeds. The diameter of the rotor would be limited by its ability to carry stress. A rotor of this construction having a large diameter and being rotated to high speeds would also pose difficulties in maintaining connectivity between the laminations and the central shaft. The stresses in a motor/generator rotor are proportional to the square of the peripheral speed, so if the diameter can be made small, the stresses can be kept low. For an energy storage flywheel, the goals are opposite: the peripheral speed is preferably made large for effectively storing a large amount of energy with the rotor.
The invention provides an inductor alternator flywheel system that has increased efficiency and is more compact than previous devices. The higher efficiency enables the system to operate with fully activated field power, allowing it to supply power without interuption in the event of a power failure in the grid. The higher efficiency also provides the potential for the system to operate at higher speed and higher power. The inductor alternator flywheel system uses a hollow steel cylindrical flywheel with the inductor alternator in the center. The bore of the hollow cylinder contains inwardly projecting teeth or castellations, which serve as the poles of the inductor alternator. A central cylinder, preferably having at least a partially laminated construction, cooperates with the rotor bore to form the stator. Armature coils are located radially between the central cylinder and rotor bore. At least one separate field coil provides the flux for alternator operation. The construction allows current to this field coil to be kept fully energized when the flywheel is rotating, even in standby operation. The flywheel is accelerated to full speed and kept at that speed by regulating the power from a synchronous inverter that drives the armature coils. During an interruption of primary power, the flywheel system instantly supplies full load power to the output without the time delay that occurs in prior art systems while the low stand-by current in a field coil is increased to establish the fill field flux necessary to generate fill power. As the speed is reduced from energy being extracted, the power to the field coil is further increased to maintain a constant output voltage until most of the energy is extracted.
Building a steel flywheel with a central hole causes the hoop direction stress to increase by a factor of two or greater. Metal flywheels that are constructed to be solid, without a central hole, have the lowest internal stress and can be operated at the highest rotational speed. Nevertheless, use of a hollow center in this invention is preferable, as will be shown. Prior inductor alternator flywheel systems are operated at relatively low stress levels; it is the system efficiency, and not the flywheel integrity, that is the main factor limiting performance. Placing the inductor alternator inside the bore allows a larger majority of the flywheel mass to be located at a large diameter, thereby maximizing the inertia per size and weight of the flywheel. With the inductor alternator integrated in the center, the system is also made more compact.
Further adding to the benefits of the invention, the construction provides the potential for maximizing efficiency. Because the flywheel is made as a hollow cylinder, the inductor alternator and flywheel energy storage functions can be partially separated such that their performances can be improved independently. The inductor alternator rotor with inwardly projecting poles can be made as a liner that is inserted into the bore of the flywheel rim, so the flywheel rim can be made from a high strength steel and heat treated to a high hardness while the liner is made from a lower magnetic loss material that has lower strength. The flywheel rim can be used to reinforce the liner for high speed rotation and the liner allows the inductor alternator to efficiently operate at high speeds. The liner can be interference fit into the bore of the flywheel rim by shrink fitting or press fitting which drives the liner initially into compression. During high-speed rotation, the stresses in the liner are thus kept low and acceptable for the liner material. Although the flywheel rim outside the liner may be used as part of the magnetic circuit, the flux through the rim is nearly uniform after passing through the liner and hence does not induce significant eddy currents that would be induced by a fluctuating magnetic flux, so losses are minimized.
In one embodiment of the invention, the liner is contrasted from multiple axially stacked layers that reduce the eddy currents in the pole teeth. The layers of the liner can be constructed of steel and separated by layers of electrically insulating material. For the lowest magnetic losses, the axial layers of the liner are constructed of very thin layers, preferably laminated steel. The thin laminations are electrically insulated by oxidizing or coating with enamel type coatings prior to stacking. The thinner the laminations for a given liner material, the lower the losses in the flywheel system but a balance must be made based on the costs.
In yet another embodiment of the invention, the hoop stress in the liner is further reduced to near zero stress by fabrication the liner from multiple individual segments about its circumference. This precludes the development of hoop stresses in the liner and can also reduce the cost of the liner. Instead of having to stamp rings where the center material is wasted, better material usage can be achieved by stamping the liner elements as arc segments.
The flywheel/inductor alternator rotor can be supported for rotation in the flywheel system by several different methods. One method involves connecting smaller diameter bearings in a hub that is mounted in the bore of the flywheel rim, and supporting those bearings on a central fixed shaft. To prevent shorting out the flux from the field coil through the hubs, they are preferably constructed from a non-ferromagnetic material such as aluminum or they contain a section of high reluctance for the field flux. The hubs can be interference assembled with the rim, or use sliding or bending joints to maintain connection with the rim at high speed. The inductor alternator flywheel system can employ upper and lower field coils or alternatively only one field coil can be used. In one version of the invention, the weight of the flywheel rotor is born by permanent magnets attached to the rotor that are in repulsion with stationary permanent magnets. Another version uses a central hub, which divides the rotor in two sections along its axial length. Upper and lower armature coils are employed, creating an upper and lower inductor alternator. In another version of the invention, the center cylinder is attached to the flywheel and rotates with it, also serving as the shaft for journaling between upper and lower bearings. The shaft is again attached to the flywheel rim using a high reluctance hub.
In one configuration of the invention the flywheel rotor is supported using full levitation magnetic bearings. The flywheel is supported radially by upper and lower radial magnetic bearings. A separate axial magnetic bearing can be used lift the flywheel weight. However in one embodiment of the invention, and this embodiment could be used with other types of inductor alternator flywheel systems, the field coil or coils are used to provide the active axial magnetic bearing in a fully levitated system. Although prior art systems used the field coils to reduce the weight of the flywheel that is carried by mechanical bearings, these systems used load force to provide feedback or were set as a constant lift force by supplying a constant current. For a fully levitated flywheel rotor, a constant current does not create a dynamically stable system. An axial position sensor is preferably used to sense the rotors position and to provide feedback for axial levitation. Because the field coil in an inductor alternator system is preferably wound to be very large and have a large inductance so that a high field can be created with low power consumption, the response time of the coil is very slow. The response time was not important for systems that only removed a portion of the load because the rotor position was essentially fixed and not dynamically unstable. However, for a fully levitated rotor, controlling the current to the field coils to achieve axial levitation would be extremely difficult if not impossible. The field coils do set up large axial direction fluxes that generate enough force to levitate the flywheel. The invention allows for these field coils to provide levitation force by replacing a single field coil with two coils. One coil is very large and it provides the flux for the inductor alternator operation. This flux is used as a bias flux for the magnetic bearing. The second coil is smaller, with a much lower inductance and faster response time, and it is used to provide the axial position control using feedback from the axial position sensor. The control force from the smaller coil is amplified by having the bias flux since the force is proportional to the square of the flux density.